Reduction in reflection from glass



Search Room June 20, 1944. a. 1.. DIMMICK REDUCTION IN REFLECTION FROM GLASS Filed Aug 29, 1941 ca F; .41 (F, 0/0 /9 0 Znvcnhr BLEHHkDImmICK dttorncg Search Room Patented June 20, 1944 REDUCTION IN REFLECTION FROM GLASS Glenn L. Dimmick. Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application August 29, 1941, Serial No. 408,807

6 Claims.

This invention relates to a reflection reducing coating for the surface of glass or similar materials.

It has been known for some time that if an optical medium, such as glass was coated with transparent material having a thickness of an odd number of quarter wave lengths of light and an index of refraction approaching the square root of that of the optical medium the surface reflectign for light of the Wave length 10 terial is evaporated on glass of index 1.51? the from which t thickness of the coating was reflected light is reduced to approximately twenty computed m b reduced t a minimum percent of its original value. This evaporation Various materials have been proposed for such is accomplished in the ua manner by heating coatings including, for example, the fatty acid 1Jhe materiel Packed n a ciled filament in ap- 1t of t lk earths and t fluoride of propriate relation to the surface to be coated s of t m n t In my application in an evacuated chamber. In the drawing the Serial No. 348,815, filed July 31, 1940, I disclosed glass is represented at Ill and the layer of gearkthe fact that a mixture of calcium fluoride and u ite evaporated on the glass is represented 1, aluminum oxide produced a coating approachat ing the optical properties of calcium fluoride but 0 The film is then aged by baking t for Sevhaving far more desirable mechanical properties eral hours at about When it is extremely in that the coating was more adherent and more hard and 111501111919 in Water- The Solubility f resistant toabrasion than those theretofore the film in nitric id, hydrochloric acid or sulknowm phuric acid is extremely low.

Th present, i ti pertains t an improved The surface friction of this evaporated film is coating which is both optically and mechanically extremely low which is a distinct advantage as superior t that of my rli r application i that it makes it more diflicult to scratch the film when the material more nearly approaches the re- W p n h Surfaces- An h d n e f the quired optical conditions, is extremely hard and particular material is that it acts as its own tenacious, and provides a very slippery surface et r rin he proc s f v pora ion. In which tends to reduce the probability of methe evaporation of the prior materials, such as chanical damage. calcium fluoride or the mixture of calcium fluo- These ends are accomplished by the use of an oride and aluminum oxide above referred to, it hydroxyfluoride of calcium and aluminum which is necessary to obtain a vacuum of about 0.2 may be found naturally as the mineral gearkmicron mercury column in order to evaporate a sutite. hard uniform film. In the evaporation of geark- One object of the invention is to provide an sutite the evaporation may be started at a presimproved reflection reducing coating for optical sure of approximately 0.6 microns and the preselements. sure then immediately falls to approximately Another object of the invention is to provide two-tenths of a micron as soon as the gearkan improved coating for glass which will adhere sutite begins to boil. The cause of this action thereto with greater tenacity than the coatings is not known but it results in cutting the evapoheretofore known. rating time in less than half, as in the ordinary Another object of the invention is to provide procedure using a large oil molecular pump with reflection reducing coating for optical media an oil fore pump it requires from 10 t 15 which is permanent and not readily damaged. utes to accomplish this same pressure reduction.

Other and incidental objects of the invention Gearksutite is described in more detail in will be apparent to those skilled in the art from Bulletin No. 20 of the U. S. Geological Survey a reading or the following specification and an n Mineralogy f th Rocky Mountains, inspection of the accompanying drawing in I claim as my ti which: 1. An optical element having a refracting sur- The sin e figure of drawings represents a reface and a reflection reducing coating thereon flcctlcn red i at according to the p having a thickness of approximately one quarter ent invention as applied on the surface of a wavelength of t light for which maximum piece of glass. reflection reduction is to be .produced and con- The mineral gearksutite has the formula CaFaAl (F,OH) 3H2O and occurs naturally in Greenland and in Colorado. The index of refraction of the material along the several crystal axes is between 1.45 and 1.46 and the index of refraction of the evaporated material is approximately 1.3.

When a quarter wave length film of this masisting of evaporated and deposited gearksutite.

2. An optical element having a retracting surface and a reflection reducing coating thereon having a thickness of approximately an odd number of quarter wavelengths of the light for which maximum reflection reduction is to be produced and consisting of evaporated and deposited gearksutite.

3. An optical element having a refracting surface and a reflection reducing coating thereon having a thickness 01' approximately one quarter wave length of the light for which maximum reflection reduction is to be produced and consisting of evaporated and deposited hydroxyfluoride of calcium and aluminum.

4. An optical element having a retracting surface and a reflection reducing coating thereon having a thickness of approximately an odd number of quarter wavelengths of the light for which maximum reflection reduction is to be produced and consisting of evaporated and deposited hydroxyfluoride of calcium and aluminum.

5. An optical element having a refracting surface and a reflection reducing coating thereon having a thickness of approximately one quarter wavelength of the light for which maximum reflection reduction is to be produced and consisting of evaporated and deposited 6. An optical element having a retracting surface and a reflection reducing coating thereon having a thickness of approximately an odd number of quarter wavelengths of the light for which maximum reflection reduction is to be produced and consisting of evaporated and deposited CaFz-AMEOH) :rno.

GLENN L. DIMMICK. 

